.TH std::gslice 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::gslice \- std::gslice

.SH Synopsis
   Defined in header <valarray>
   class gslice;

   std::gslice is the selector class that identifies a subset of std::valarray indices
   defined by a multi-level set of strides and sizes. Objects of type std::gslice can
   be used as indices with valarray's operator[] to select, for example, columns of a
   multidimensional array represented as a valarray.

   Given the starting value s, a list of strides i
   j and a list of sizes d
   j, a std::gslice constructed from these values selects the set of indices k
   j=s+Σ
   j(i
   jd
   j).

   For example, a gslice with starting index 3, strides {19,4,1} and lengths {2,4,3}
   generates the following set of 24=2*4*3 indices:

   3 + 0*19 + 0*4 + 0*1 = 3,
   3 + 0*19 + 0*4 + 1*1 = 4,
   3 + 0*19 + 0*4 + 2*1 = 5,
   3 + 0*19 + 1*4 + 0*1 = 7,
   3 + 0*19 + 1*4 + 1*1 = 8,
   3 + 0*19 + 1*4 + 2*1 = 9,
   3 + 0*19 + 2*4 + 0*1 = 11,
   ...
   3 + 1*19 + 3*4 + 1*1 = 35,
   3 + 1*19 + 3*4 + 2*1 = 36

   It is possible to construct std::gslice objects that select some indices more than
   once: if the above example used the strides {1,1,1}, the indices would have been {3,
   4, 5, 4, 5, 6, ...}. Such gslices may only be used as arguments to the const version
   of std::valarray::operator[], otherwise the behavior is undefined.

.SH Member functions

   constructor   constructs a generic slice
                 \fI(public member function)\fP
   start         returns the parameters of the slice
   size          \fI(public member function)\fP
   stride

std::gslice::gslice

   gslice()                                                                \fB(1)\fP
   gslice( std::size_t start, const std::valarray<std::size_t>& sizes,     \fB(2)\fP
                              const std::valarray<std::size_t>& strides );
   gslice( const gslice& other );                                          \fB(3)\fP

   Constructs a new generic slice.

   1) Default constructor. Equivalent to gslice(0, std::valarray<std::size_t>(),
   std::valarray<std::size_t>()). This constructor exists only to allow construction of
   arrays of slices.
   2) Constructs a new slice with parameters start, sizes, strides.
   3) Constructs a copy of other.

.SH Parameters

   start   - the position of the first element
   sizes   - an array that defines the number of elements in each dimension
   strides - an array that defines the number of positions between successive elements
             in each dimension
   other   - another slice to copy

std::slice::start, size, stride

   std::size_t start() const;                 \fB(1)\fP
   std::valarray<std::size_t> size() const;   \fB(2)\fP
   std::valarray<std::size_t> stride() const; \fB(3)\fP

   Returns the parameters passed to the slice on construction - start, sizes and
   strides respectively.

.SH Parameters

   \fI(none)\fP

.SH Return value

   The parameters of the slice -- start, sizes and strides respectively.

.SH Complexity

   Constant.

.SH Example

   Demonstrates the use of gslices to address columns of a 3D array:


// Run this code

 #include <iostream>
 #include <valarray>

 void test_print(std::valarray<int>& v, int planes, int rows, int cols)
 {
     for (int r = 0; r < rows; ++r)
     {
         for (int z = 0; z < planes; ++z)
         {
             for (int c = 0; c < cols; ++c)
                 std::cout << v[z * rows * cols + r * cols + c] << ' ';
             std::cout << "  ";
         }
         std::cout << '\\n';
     }
 }

 int main()
 {
     std::valarray<int> v = // 3d array: 2 x 4 x 3 elements
         {111,112,113 , 121,122,123 , 131,132,133 , 141,142,143,
          211,212,213 , 221,222,223 , 231,232,233 , 241,242,243};
     // int ar3d[2][4][3]
     std::cout << "Initial 2x4x3 array:\\n";
     test_print(v, 2, 4, 3);

     // update every value in the first columns of both planes
     v[std::gslice(0, {2, 4}, {4 * 3, 3})] = 1; // two level one strides of 12 elements
                                                // then four level two strides of 3 elements

     // subtract the third column from the second column in the 1st plane
     v[std::gslice(1, {1, 4}, {4 * 3, 3})] -= v[std::gslice(2, {1, 4}, {4 * 3, 3})];

     std::cout << "\\n" "After column operations:\\n";
     test_print(v, 2, 4, 3);
 }

.SH Output:

 Initial 2x4x3 array:
 111 112 113   211 212 213
 121 122 123   221 222 223
 131 132 133   231 232 233
 141 142 143   241 242 243

 After column operations:
 1 -1 113   1 212 213
 1 -1 123   1 222 223
 1 -1 133   1 232 233
 1 -1 143   1 242 243

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to          Behavior as published              Correct behavior
   LWG 543 C++98      it was unclear whether a default        it is usable (as an empty
                      constructed generic slice is usable     subset)

.SH See also

   operator[]   get/set valarray element, slice, or mask
                \fI(public member function)\fP
   slice        BLAS-like slice of a valarray: starting index, length, stride
                \fI(class)\fP
   gslice_array proxy to a subset of a valarray after applying a gslice
                \fI(class template)\fP
